Endogenous Zn2+ is required for the induction of long-term potentiation at rat hippocampal mossy fiber-CA3 synapses

The functional role of the abundant Zn(2+) found in some hippocampal synapses has been an enigma. We show here, using N-[6-methoxy-8-quinolyl]-P-toluenesulfonamide (TSQ) staining, that chelatable-Zn(2+) can be removed from hippocampal synaptic boutons using dietary depletion or with Zn(2+) chelators. A chronic dietary deficiency of bouton Zn(2+) resulted in the impairment of long-term potentiation (LTP) at mossy fiber-CA3 synapses. The averaged normalized fEPSP slope 30 min after tetanus was 209 +/- 28% of baseline value in control (mean +/- SEM, n = 10), and 118 +/- 12% in Zn(2+)-deficient rats (mean +/- SEM, n = 12, P < 0.01). In the deficient rats with Zn(2+) supplements, mossy fiber LTP returned to normal levels. The acute depletion of bouton Zn(2+) in the hippocampal slice with membrane-permeable Zn(2+) chelators, dithizone, or diethyldithiocarbamic acid (DEDTC) blocked the induction of mossy fiber LTP. The mean amplitudes of EPSCs after tetanus were 194 +/- 22% of baseline value in control (n = 5), compared to 108 +/- 14% in dithizone (n = 6) and 101 +/- 12% in DEDTC (n = 5). The averaged value of LTP, at the associational commisural fiber-CA3 synapses, was 193 +/- 20% in the control (n = 6), compared to 182 +/- 21% (n = 6, P > 0.1) in the presence of dithizone. The blockade of mossy fiber LTP by dithizone was reversible after washout. In addition, normal LTP could be induced by tetanus if exogenous Zn(2+) was applied immediately following dithizone. Our results indicate that the endogenous Zn(2+) is specifically required for LTP induction at the mossy fiber input into CA3 neurons.